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SubjectRe: [PATCH v10 5/6] x86/split_lock: Handle #AC exception for split lock
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> On Nov 20, 2019, at 5:45 PM, Fenghua Yu <fenghua.yu@intel.com> wrote:
>
> Currently Linux does not expect to see an alignment check exception in
> kernel mode (since it does not set CR4.AC). The existing #AC handlers
> will just return from exception to the faulting instruction which will
> trigger another exception.
>
> Add a new handler for #AC exceptions that will force a panic on split
> lock for kernel mode.
>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> ---
> arch/x86/include/asm/traps.h | 3 +++
> arch/x86/kernel/cpu/intel.c | 2 ++
> arch/x86/kernel/traps.c | 22 +++++++++++++++++++++-
> 3 files changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index b25e633033c3..0fa4eef83057 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -172,4 +172,7 @@ enum x86_pf_error_code {
> X86_PF_INSTR = 1 << 4,
> X86_PF_PK = 1 << 5,
> };
> +
> +extern bool split_lock_detect_enabled;
> +
> #endif /* _ASM_X86_TRAPS_H */
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 2614616fb6d3..bc0c2f288509 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -32,6 +32,8 @@
> #include <asm/apic.h>
> #endif
>
> +bool split_lock_detect_enabled;
> +
> /*
> * Just in case our CPU detection goes bad, or you have a weird system,
> * allow a way to override the automatic disabling of MPX.
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 4bb0f8447112..044033ff4326 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -293,9 +293,29 @@ DO_ERROR(X86_TRAP_OLD_MF, SIGFPE, 0, NULL, "coprocessor segment overru
> DO_ERROR(X86_TRAP_TS, SIGSEGV, 0, NULL, "invalid TSS", invalid_TSS)
> DO_ERROR(X86_TRAP_NP, SIGBUS, 0, NULL, "segment not present", segment_not_present)
> DO_ERROR(X86_TRAP_SS, SIGBUS, 0, NULL, "stack segment", stack_segment)
> -DO_ERROR(X86_TRAP_AC, SIGBUS, BUS_ADRALN, NULL, "alignment check", alignment_check)
> #undef IP
>
> +dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
> +{
> + unsigned int trapnr = X86_TRAP_AC;
> + char str[] = "alignment check";
> + int signr = SIGBUS;
> +
> + RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> +
> + if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) == NOTIFY_STOP)
> + return;
> +
> + if (!user_mode(regs) && split_lock_detect_enabled)
> + panic("Split lock detected\n");

NAK.

1. Don’t say “split lock detected” if you don’t know that you detected a split lock. Or is this genuinely the only way to get #AC from kernel mode?

2. Don’t panic. Use die() just like every other error where nothing is corrupted.

And maybe instead turn off split lock detection and print a stack trace instead. Then the kernel is even more likely to survive to log something useful.

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